iOS Designated Initializers : Using NS_DESIGNATED_INITIALIZER

The use of NS_DESIGNATED_INITIALIZER is nicely explained in http://useyourloaf.com/blog/2014/08/19/xcode-6-objective-c-modernization.html:

The designated initializer guarantees the object is fully initialised
by sending an initialization message to the superclass. The
implementation detail becomes important to a user of the class when
they subclass it. The rules for designated initializers in detail:

• A designated initializer must call (via super) a designated
  initializer of the superclass. Where NSObject is the superclass this
  is just [super init].
• Any convenience initializer must call another
  initializer in the class – which eventually leads to a designated
  initializer.
• A class with designated initializers must implement all
  of the designated initializers of the superclass.

As an example, if your interface is

@interface MyClass : NSObject
@property(copy, nonatomic) NSString *name;
-(instancetype)initWithName:(NSString *)name NS_DESIGNATED_INITIALIZER;
-(instancetype)init;
@end

then the compiler checks if the (convenience) initializer init calls
the (designated) initializer initWithName:, so this would cause a warning:

-(instancetype)init
{
    self = [super init];
    return self;
}

and this would be OK:

-(instancetype)init
{
    self = [self initWithName:@""];
    return self;
}

In Swift the rules about designated and convenience initializers are even more strict,
and if you mix Objective-C and Swift code, marking the designated Objective-C initializers helps the compiler to enforce the rules.

For example, this Swift subclass would cause an compiler error:

class SwClass: MyClass {
    var foo : String
    init(foo : String) {
        self.foo = foo
        super.init()
    }
}

and this would be OK:

class SwClass: MyClass {
    var foo : String
    init(foo : String) {
        self.foo = foo
        super.init(name: "")
    }
}